We have recently demonstrated that the thyroid gland of the sheep contains a soluble protein with a high binding affinity for serotonin. partial purification of the protein has been carried out by ammonium sulfate fractionation. We have also determined that the sheep's thyroid contains a relatively high concentration of endogenous serotonin. Since parafollicular cells are known to contain serotonin, therefore, it is possible that parafollicular cells are the source of both endogenous serotonin and the serotonin-binding protein in the thyroid gland of the sheep. In order to test this hypothesis, we propose to carry out a biochemical and stereological electron microscopic study to localize serotonin, serotonin-binding protein and parafollicular cells in the thyroid gland of the sheep.